1. Field of the Invention
The present invention relates to mine roof bolts, and more particularly relates to mine roof bolts constructed of pretensioned, multi-strand steel cable.
2. Description of the Prior Art
In the art of mine roof support, there are two major categories of bolt systems wherein mine roof bolts are anchored in bore holes drilled in the mine roof, the bolts' purpose being to reinforce the unsupported rock formation above the mine roof. These two categories of mine roof bolt systems are: (1) tension-type systems, and (2) passive-type systems. In each system, it is common practice to, first, drill a hole through the mine ceiling into the rock formation above to a depth appropriate for the type of rock formation to be supported. A mine roof bolt and roof plate are then anchored in the bore hole to support the mine roof and maintain the rock formation in place.
In tension-type mine roof bolt systems, an expansion shell type anchor is installed on the end of the bolt. The bolt and expansion shell anchor are inserted up into the bore hole until the roof plate is against the mine roof. The bolt is then rotated to thread a tapered plug section of the expansion shell down toward the bolt head, in order to expand the jaws of the expansion shell against the interior wall of the bore hole to thereby hold the mine roof bolt in place within the bore hole.
In passive-type mine roof bolt systems, the passive-type bolt is not attached to an expansion shell or similar device at the free (upper) end of the bolt, but rather is retained in place within the rock formation by a rapid-curing resin material that is mixed in the bore hole as the bolt is rotated and positioned within the bore hole. In theory, the resin bonds the bolt to the rock formation along the total length of the bolt within the bore hole in the rock formation. It is also common practice to use resin with a tensiontype mine roof bolt to retain the bolt within the mine roof bore hole.
In these passive-type mine roof bolt systems, one or more resin cartridges are inserted into the bore hole prior to (ahead of) the mine roof bolt. Forcing the mine roof bolt into the bore hole while simultaneously rotating the bolt ruptures the resin cartridge(s) and mixes the two resin components within the annulus between the bolt shank and bore hole wall. Ideally, the resin mixture totally fills the annulus between the bolt shank and bore hole wall along the total length of the bolt and bore hole. The resin mixture penetrates the bore hole wall and into the surrounding rock formation to adhere the bolt to the rock formation.
When extremely long mine roof bolts are necessary, it is common practice to attach two or more bolt sections together by couplers to result in a "roof bolt" of sufficient length appropriate for the particular type of rock formation. These couplers between bolt sections, being of a larger diameter than the bolt shanks, prevent the mixed resin from flowing downwardly (resin return) within the bore hole annulus from the first (upper) bolt section to the lower section(s). Therefore, the anchoring of the bolt to the bore hole wall within the rock formation is, effectively, only along the length of the first (upper) bolt section wherein the resin totally fills the annulus between the bolt section and the bore hole wall.
To alleviate this problem, it has been common practice simply to drill a larger bore hole in the rock formation that will enable the resin to flow around the coupler(s) as the bolt is being rotated within the bore hole to mix the resin. Although this does effect the desired result (resin return around the coupler(s) within the annulus between the bolt shank and bore hole wall), it creates another problem that, depending on the type of rock formation, may be more dangerous than the problem that is corrected by a larger bore hole. Specifically, the bonding effectiveness of the resin bonding material to hold the mine roof bolt in place within the bore hole is considerably weakened by virtue of the increased distance between the bolt shank and bore hole wall, and the sheer volume of resin material necessary to totally fill the annulus with the resin bonding material. Additionally, by virtue of their specific makeups, mine roof rock formations that actually require long (fifteen feet or longer) mine roof bolts are more susceptible to movement and shifting within the rock formation, than are more solid rock formations that require only shorter mine roof bolts.
Another common problem with using mine roof bolt sections coupled together in such rock formations that require longer (coupled) mine roof bolts, this shifting of the rock formation (shear) causes the bolt couplers to fracture. When this happens, of course, the effective holding length of the mine roof bolt is instantly decreased. In many instances, there is no or very little resin adhesive material around the broken bolt shank to help stabilize the rock formation. Therefore, in almost all instances, this shortened mine roof bolt is ineffective to safely prevent the mine roof rock formation from further shifting and potential collapse.
It is therefore an object of the present invention to provide an improved mine roof bolt that does not require an oversized mine roof bore hole in order to effect full and complete resin return within the annulus between the bolt shank and bore hole wall along the total length of the bolt shank.
It is another object of the present invention to provide an improved mine roof bolt that is available in various lengths without the use of bolt shank couplers that are susceptible to fracture when the mine roof rock formation shifts.
It is a further object of the present invention to provide an improved mine roof bolt having an inherently rough outer surface that aids in effecting complete mixture of the resin bonding material, and also includes crevices within the mine roof bolt shank that permit penetration of the resin bonding material int the bolt shank for more effective resin bonding thereto.
It is a still further object of the present invention to provide an improved mine roof bolt that will easily bend for installation into a bore hole that is considerably deeper than the height of the mine at the installation location, and will also bend with a shifting rock formation, and fully retain its bonding within the rock formation along the total length of the mine roof bolt without breaking when the rock formation shifts.